The use of electrically heated wires and films as constant temperature anemometer transducers is well known in the prior art. In such devices a heated resistance element serves as a sensing element and its physical geometry is used to define its spatial response to impinging fluid flow. Most widespread use has been in the measurement of airflow. The sensing element has a temperature coefficient of resistance and is maintained at constant resistance and, thus, constant temperature while it is operated as part of a feedback controlled electrical bridge circuit. An example of such a prior art constant temperature anemometer circuit is illustrated in FIG. 1 wherein a single sensing element is shown at 10 and forms one arm of a four arm Wheatstone bridge which is completed by resistances 11, 12 and 13. Differential amplifier 14 is connected to the bridge at points 15 and 16 in order to determine bridge balance or bridge error signal and amplifier 14 output 17 is fed back to the bridge in order to provide bridge excitation. For clarity's sake power supply connections are not shown in this figure and those following. The resulting signal output 17 is unipolar and is markedly non-linear, containing three components. These are an approximate fourth root term as a function of mean flow, a d-c or constant term which is the zero flow quiescent heating signal, and a turbulence component which results from fluctuations in the flow. Examples of single-ended unipolar constant temperature anemometer transducers, together with bridge operating circuits therefor, are shown in U.S. Pat. Nos. 3,220,225; 3,352,154; 3,363,462; 3,900,819; 3,991,624; 4,373,387; 4,503,706 and 4,523,462.
U.S Pat. Nos. 3,220,255 and 3,363,462 describe the non-linearity of the single-ended constant temperature anemometer bridge output signal in detail and teach us means for analog signal processing in order to provide a linearized output signal.
A prior art example of bipolar operation of a constant temperature anemometer transducer is shown in FIG. 2 wherein two series connected resistive sensing elements 10a and 10b produce a bipolar non-linear output. Two sensing elements 10a and 10b together with resistors 18 and 19 form a second Wheatstone bridge in place of the single sensing element 10 of FIG. 1. A differential amplifier 22, connected to points 20 and 21, provides a composite bridge output signal 23 which contains the above mentioned non-linear mean flow term with implicit polarity sign and a turbulence component but no d-c or constant term. Examples of differential bipolar constant temperature anemometer transducers, together with bridge operating circuits therefor, are shown in U.S. Pat. No. 4,279,147 and in co-pending U.S. patent application Ser. No. 866,604 filed May 23, 1986. Examples of unipolar bridge signal with separate sign sensing output constant temperature anemometer transducers, together with bridge operating circuits therefor, are shown in U.S. Pat. Nos. 3,352,154; 3,900,819; 3,991,624; 3,995,481; 4,024,761 and 4,206,638.